Rooftop units for cooling, heating and exhausting the air of a building are very common. They are typically installed on stores, restaurants, theaters and other buildings which are not sufficiently large to warrant a central chiller of the evaporative cooling tower type. These unitary systems include a compressor packaged with an air-cooled condenser blower-and-coil and also a recirculating air blower-and-coil. Often they operate in various modes including a reverse cycle for the refrigerant so that heat can be provided by the compressor pumping heat from the outdoor air entering the building, which means that the condenser coil converts into an evaporator coil and a recirculating air coil becomes a condenser. This requires a four-way reversing valve, a defrost cycle on the outdoor air coil and relatively complicated controls, valves, refrigerant accumulators, etc. One of the advantages of such rooftop units is that they do not involve the use of gas as a fuel and therefore operate without regard to fuel shortages and gas pressure fluctuation.
A significant disadvantage of rooftop units of this sort is that their air-cooled condensers are notoriously inefficient. During peak-load conditions on hot summer afternoons the air-cooled condensers often heat up to 140 degrees F. to reject heat, thus forcing the compressors to work much harder and accomplish less cooling precisely when it is most needed. On hot summer days conventional rooftop units are among the worst non-residential contributors to utility peak loads because of this low efficiency combined with low output. Many utilities have offered subsidies to users who avoid peak loads from rooftop air conditioning units using air-cooled condensers. Levelling of peak loads with inexpensive and uncomplicated air heating and cooling equipment is one of the principal purposes of the present invention.